Inkjet imaging devices are used to print a wide range of documents using various types and colors of ink. Some printed documents are read by both humans and by other machines. For example, a check includes printed text that is both human readable and readable by automated check processing equipment. Check processing machines use Magnetic Ink Character Recognition (MICR) to identify printed characters in a check, such as routing and account numbers, quickly and accurately. The magnetic ink includes a suspension of magnetic particles, such as iron oxide, which are detectable using a magnetic field. The use of MICR printing is widespread and enables automated processing of checks and other documents even when the printed magnetic ink characters are visually obscured by stamps or other overprinting. Automated check processing machines perform high-speed character recognition using printed magnetic ink characters to identify account and routing numbers. While check processing is one application of magnetic ink printing, magnetic inks can be incorporated in a wide range of printed documents and can be used in conjunction with non-magnetic inks as well.
One challenge in using magnetic inks with inkjet printers relates to the propensity of the magnetic inks to absorb or “bleed” into porous print media such as uncoated paper. A typical magnetic ink includes a liquid solvent holding a suspension of microscopic magnetic particles. The liquid solvent and magnetic particles are ejected in the form of drops onto the paper where the solvent evaporates, leaving the magnetic particles. During the drying process, however, the solvent and magnetic particles absorb into the fiber of the paper instead of remaining concentrated near the surface of the paper.
FIG. 5 depicts a prior art inkjet 504 in the process of ejecting magnetic ink drops 508 onto a porous print medium 514. The magnetic ink drops 508 initially form a character 512 on the surface of the print medium 514 as the print medium 514 moves past the inkjet 504 in the process direction P. Over time, however, the print medium 514 absorbs some of the solvent and magnetic particles. A portion of the ink 518 from an earlier printed character 516 bleeds into the porous print medium 514, and an even greater portion 524 of a printed character 520 that was printed prior to the character 516 bleeds into the porous print medium 514. In addition to being visually undesirable, the absorbed ink spreads over a wider area of the print medium 514 and printed characters or other markings are distorted due to the absorption of the magnetic ink. The distortion may reduce the ability of automated scanning equipment to read information encoded with the magnetic ink accurately.
To reduce or eliminate bleeding, some magnetic ink printers use specially treated print media, such as wax coated papers, which reduce the porosity of the print medium to reduce or eliminate the absorption of the magnetic ink. Using coated papers, however, adds to the expense of printing documents and reduces the versatility of a magnetic ink printing system. Consequently, improvements to magnetic ink printers that improve the readability of images printed with magnetic ink would be beneficial.